An LED element (light emitting diode) is a semiconductor element that emits light by application of a voltage, and is widely used due to high brightness, long life, and a characteristic of obtaining light not containing unnecessary ultraviolet rays or infrared rays. Asa use thereof, the LED element is applied to a lighting device as well as a headlight of an automobile, a backlight of an electronic device, various displays, and the like.
The light emitted from the LED element is monochromatic light of a frequency corresponding to a bandgap of a chemical compound forming a semiconductor. Therefore, since a wavelength of the emitted light is changed in accordance with the kind of chemical compound, LED elements that emit various emitting light colors are manufactured. As the chemical compound, for example, Ga (gallium), N (nitrogen), In (indium), Al (aluminum), P (phosphorus), and the like are used.
A white LED light emitting element that is widely used for a backlight for a liquid crystal device, and a lighting device, etc., is realized by combining a semiconductor light emitting element that emits blue light and green, red, and yellow fluorescent bodies, etc. The kinds of fluorescent bodies include quantum dot fluorescent bodies (hereinafter, referred to as semiconductor quantum dots) consisting of inorganic fluorescent bodies, organic fluorescent bodies, and semiconductors.
Among these fluorescent bodies, semiconductor quantum dots are very small semiconductor particles, the maximum particle diameter of which is 50 nm or less. The semiconductor quantum dots absorb photons of energy greater than a bandgap (energy difference between a valence band and a conduction band), and emit light having a wavelength corresponding to the particle diameter thereof. That is, by having a property of absorbing light of a fixed wavelength or less and controlling the particle diameter, light of various wavelengths can be generated.
A semiconductor quantum dot shows a quantum confinement effect in a region smaller than the Bohr radius of a compound semiconductor excitation, and can realize high fluorescence efficiency. Here, the fluorescence efficiency is a ratio of the number of photons of emitted fluorescent light to the number of photons of light input in the semiconductor quantum dot. Luminescence efficiency is an index of brightness that the whole of a light emitting device can generate with fixed energy.
In a light emitting device utilizing semiconductor quantum dots, part of light emitted from a light source is converted into light of a predetermined wavelength by a quantum dot fluorescent body, and the other part is emitted as it is. These two kinds of lights are mixed and recognized as white light by human vision. Under such circumstances, a technique relating to a light emitting device capable of emitting light of a desired wavelength with use of semiconductor quantum dots is proposed (for example, refer to Patent Document 1 and Patent Document 2).
On the other hand, fluorescence efficiency of semiconductor quantum dots depends on surface characteristics such as a surface structure and surface crystallinity. A semiconductor quantum dot is a particle with a very high ratio of surface area to volume since the semiconductor quantum dot has a particle size very small as several tens of nanometers. Therefore, defects easily occur on surfaces of semiconductor quantum dots.
Defects on surfaces of semiconductor quantum dots act as various energy traps existing in a bandgap, and results in deterioration of the luminescence efficiency of the whole device. In detail, dangling bonds (unsatisfied valences of atoms) on the surfaces and atomic vacancies cause an imbalance of electric charges, and excited electrons are trapped, so that fluorescence efficiency is deteriorated.
Due to the defects on the surfaces of the semiconductor quantum dots, the fluorescence efficiency deteriorates, and light from the light source becomes difficult to be wavelength-converted. As a result, the luminescence efficiency of the whole device is deteriorated. In addition, influences of various exogenous factors may cause defects on surfaces of semiconductor quantum dots. However, at present, major causes are not clearly known.
Under these circumstances, a light emitting device in which contact between oxygen and semiconductor quantum dots is reduced on the assumption that defects on surfaces of semiconductor quantum dots are mainly caused by oxidation of the surfaces due to oxygen has been proposed (for example, refer to Patent Document 3).
In detail, Patent Document 3 describes the light emitting device 400 as shown in FIG. 9. This light emitting device 400 includes a package 401 made of a resin, a lead frame 402, and a semiconductor light emitting element 403 mounted on the lead frame.
The light emitting device 400 includes a resin portion 404 formed so as to cover the semiconductor light emitting element 403. The resin portion 404 includes semiconductor quantum dots 405 and getter particles 406 that adsorb oxygen.
In the light emitting device 400 described in Patent Document 3, the amount of oxygen that comes into contact with the surfaces of the semiconductor quantum dots 405 is reduced by adsorption of oxygen entering the resin portion 404 by the getter particles 406.